O-LevelPhysicsLight5054/21 · Oct/Nov 2016 · Q45 Marks
A lamp is positioned at the bottom of a small pool of water. The critical angle for light passing from water into air is 49°.
- (a)Explain what is meant by the term critical angle.[2]
- (b)The lamp sends light towards the surface of the pool. Fig. 4.1 shows three rays of light that are at 30°, 60° and 90° to the horizontal. [Figure 4.1] On Fig. 4.1, draw the path taken by each of the three rays after they strike the surface of the water.[3]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2016). The mark or mark type is shown in brackets.
- [B1] smallest angle for total internal reflection or angle for refraction along surface
- [B1] angle of incidence in (optically) denser medium
- [B1] vertical ray continues undeviated
- [B1] second ray (60° to horizontal) refracts away from normal into the air
- [B1] third ray reflects internally and i = r by eye not if any refracted ray
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